The invention relates to a process and a catalyst for dehydrogenation or dehydrocyclization of hydrocarbons on regenerable catalysts, consisting of chromium oxide and aluminum oxide with one or more promoters from the group of alkali metal and/or alkaline earth metal compounds and with at least one additional promoter.
It is generally known that hydrocarbons can be dehydrogenated in the presence of suitable catalysts at higher temperature. Depending on the type of hydrocarbons used, in the case of paraffins especially as a function of the chain length, either a dehydrogenation to monoolefins or diolefins or, possibly also simultaneously, a cyclization i.e. ring formation, mainly to aromatic compounds, with simultaneous cleavage of hydrogen, a so-called dehydrocyclization, takes place.
Below, by the term "dehydrogenation" are understood all reactions in which hydrogen is cleaved from hydrocarbons of any type, without regard to whether the dehydrogenation products form straight or branched chains whether they are cyclic or whether they are singly or repeatedly unsaturated.
As can be seen, for example, from U.S. Pat. No. 3,719,721, aluminum oxide-chromium oxide catalysts are used for such dehydrogenation processes.
But since catalysts with aluminum oxide as support material exhibit acid properties, which lead to undesirable isomerization or cracking reactions, these known catalysts in addition contain a small portion of an alkali metal oxide and/or alkaline earth metal oxide, which acts as a base and thus improves the selectivity of the catalyst.
As other promoters, the known catalysts contain niobium or tantalum oxide or also, as a comparison example, cerium oxide, which contributes to increasing the catalyst activity.
Despite the improved selectivity and activity of the chromium oxide catalysts, in the dehydrogenation according to the process of the above-mentioned U.S. Patent, carbon or coke results, which is precipitated on the catalyst and deactivates it continuously, which means a considerable drawback relative to economy and investment costs.
The known process further has the drawback that the selectivity or activity of the catalyst is not sufficient to keep the by-product formation values optimally low--especially to prevent multiple dehydrogenations--to obtain pure monoolefin dehydrogenation products in a suitable feedstock stream.